Why Google Slipped Site from 1st Page to other Pages

I am working as a SEO for the last 5 years but unable to understand the GOOGLE strategies regarding the site listings my site is on the FIRST PAGE of the Google results but my site slipped to 50th page I dont understand why, please HELP me regarding this serious issue.

It's difficult to come up with anything other than what has already been suggested without knowing what the site is and what the keyword is that it has slipped in the rankings for.
Also is it a new site, and how long was it on the first page before it dropped? To avoid receiving suggestions for things that you may already have looked at, maybe you could say what you have eliminated as being the possible reasons, and why.

It can be google dance also or may be you have been building backlinks on irrelevant sites which got into google's notice a bit late. Relevancy and content is the major factor for google never skip it.

Hi,
I think, it might be Google dance because dropping the position from top position to fifth page is not a usual thing.
My suggestion to you is, just review the content and add some fresh with the help of content writer. Also, increase the link building stuff, this will manage the part of backlinks and do active SMO for traffic. You will get your position back soon.

Hi,
I think, it might be Google dance because dropping the position from top position to fifth page is not a usual thing.

I thought the Google Dance theory died out a few years ago? My understanding is that it was related to what were then (roughly) monthly updates to ranking positions, which were applied over a few days so that a site's position could "dance" around until all the updates were applied. But now that Google updates all the time, it is no longer a factor.